PROJECT SUMMARY: Mammalian aging is associated with reduced tissue regeneration due to declining function of tissue-specific stem cells. In the blood system, hematopoietic stem cell (HSC) aging is accompanied by an expansion of myeloid-biased HSCs with reduced self-renewal functions. Aging of HSCs and their niches likely contributes to aged-related hematologic malignancies such a myelodysplastic syndromes or acute myelogenous leukemia. Whether the aging microenvironment drives the phenotypic features of aging HSCs, however, remains unclear. In the healthy bone marrow of young mice, our prior studies show that niches cells containing mesenchymal stem cell (MSC) activity, marked by Nestin expression, are innervated and regulated by the sympathetic nervous system (SNS). Preliminary data supporting this grant application have revealed marked alterations in the vasculature of aged bone marrow, associated with loss of SNS innervation, and increased Nestin+ cell numbers. In addition, we find that the generation of reactive oxygen species (ROS) in endothelial and perivascular cells oscillates in a circadian manner, and that these oscillations are impaired in aged bone marrow. Aged bone marrow endothelial cells also exhibit significant alterations in glucose metabolism, that are regulated by SNS-derived signals. Based on these preliminary data, we propose to explore the hypothesis that the loss of SNS nerve signals contributes to the hematopoietic aging phenotype via regulation of the vascular- associated niche cells. This hypothesis will be tested in three aims wherein: Specific Aim 1 will investigate the function of ?-adrenergic signals in the aging HSC niche using pharmacological and genetic approaches to modulate the sympathetic tone. In Specific Aim 2, we will investigate how SNS-enabled circadian rhythms and ROS homeostasis alter MSCs and niche function to regulate niche structure and HSC aging phenotypes. Specific Aim 3 will investigate the role of SNS innervation in age-related alterations of vascular metabolism in the HSC niche. The proposed studies will shed new light on the contributions of the aging niche on HSC function, and will help to devise new therapeutic strategies to prevent or improve the course of age-associated hematopoietic diseases.#